My ds is on this lesson as well, but is a bit older than yours (would’ve never attempted this at your ds age). We used a different program (spiral) before MUS and at some point over a year ago, maybe 2, I realized that math had too much going on (order of operation took awhile to figure out) and he is easily overwhelmed. The mastery approach works for him. Not saying your ds is having any issues, of course.
I have had a great time, even “a-ha” moments (like with other programs, I’m sure) with the blocks and double-digit multiplication (and I have never had issues before, all my facts are memorized!). I use the blocks with him on almost every problem and then we work it out the “old school” way. And, we talk about the what is really being multiplied, not just the digits but their place value. I’ll have to do this for awhile with him, but that’s okay 🙂
I do love using the blocks for him to “see” the answer by just looking, not counting, and for him to “see” how much the sum really is even when the digits are on the low-end (1, 2, 3, etc.).
I’ll go look at the last lesson and try to remember what made the blocks so helpful. Obviously it could be just my ds who needs the visual and full-on explanation, every time, and that’s why it works and we use them. Your ds could be just the opposite and they’re of no use. I don’t really know about that.
HTH